1. Field of the Invention
The invention in general relates to miniature switches, and more particularly, to a MEMS switch useful in radar and other microwave applications.
2. Description of Related Art
A variety of MEMS (microelectromechanical systems) switches are in use, or proposed for use, in radar, as well as other high frequency circuits for controlling RF signals. These MEMS switches are popular insofar as they can have a relatively high off impedance and a relatively low on impedance, with a low off capacitance, leading to desirable high cutoff frequencies and wide bandwidth operation. Additionally, the MEMS switches have a small footprint and can operate at high RF voltages.
Many of these MEMS switches generally have electrostatic elements, such as opposed pull down control electrodes, which are attracted to one another upon application of a DC control signal. One of these DC control electrodes is on a substrate and an opposing electrode, having a dielectric coating, is positioned on the underside of a moveable bridge above the substrate. Upon application of the DC control signal the bridge is drawn down and an electrical contact on the underside of the bridge completes the electrical circuit between first and second spaced apart RF conductors on the substrate.
As will be described, for this type of design there is a possibility of stiction. Stiction is a condition wherein a charge is built up in the dielectric upon touching the opposed control electrode. When the control voltage is removed there may be enough charge built up such that there is still an attraction and the switch will remain closed, even though it is supposed to be open. Further, under such condition, at the point of closure of the control electrodes an ultra-high field exists which can lead to contact erosion.
It is an object of the present invention to provide a MEMS switch which eliminates the possibility of stiction. It is a further object to provide a MEMS switch which is highly reliable, has low RF losses and a high operating bandwidth.
A MEMS switch is provided which has a substrate member with first and second spaced-apart conductors deposited on the substrate. A bridge structure, including a central stiffener portion, is disposed above the substrate and has a plurality of flexible arms connected to respective ones of a plurality of support members. At least one control electrode is deposited on the substrate for receiving a DC control signal to activate the switch to a closed position. The bridge structure has an undersurface including at least one metallic area for forming an opposed electrode portion facing the control electrode, for electrostatic attraction upon application of the DC control signal. The bridge structure, upon application of the DC control signal, is drawn down, by the electrostatic attraction, to complete an electrical circuit between the first and second conductors. The central stiffener portion is of a material to resist bending in a manner that, when said bridge structure is drawn down completing the electrical circuit, there is no contact between the control electrode and the opposed electrode portion. Additionally, the switch is fabricated such that there is no dielectric material in the area of the opposed electrode facing the control electrode.
Further scope of applicability of the present invention will become apparent from the detailed descriptions provided hereinafter. It should be understood, however, that the detailed descriptions and specific examples, while disclosing the preferred embodiments of the invention, is provided by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art, from the detailed description.